The present invention relates to a novel composite fiber which is a blend of a specific flame-retardant fiber and at least one common fiber and has an excellent flame-retardant property and other properties required for common fibers such as good hand touchness and hygroscopicity. Particularly, the invention relates to a composite fiber comprising a matrix fiber whch is made of a chlorine-containing polymer and a partially acetalized polyvinyl alcohol as main ingredients and contains as flame-retarding agents a specific tin compound and antimony compound in a specific ratio, and at least one fiber selected from the group consisting of polyester fiber, acrylic fiber and cotton fiber. Hereinafter, the term "matrix fiber" is intended to mean a flame-retardant fiber which is made of a halogen-containing polymer and polyvinyl alcohol as main polymer ingredients. Matrix fiber is also known as "polychlal fiber".
Recently there has been a strong demand for the flame-retardation of fibrous goods such as clothes and interior decorations. Heretofore, researches as to the flame-retardation of fibers have been directed to the flame-retardation of each one kind of fiber such as polyester fiber, acrylic fiber or viscose fiber, and the flame-retardation of composite fibers prepared by blending two or more different kinds of fibers has been researched scarcely.
However, it is almost impossible to produce final products having all properties required by consumers by employing one kind of flame-retardant fiber and therefore it is necessary to blend two or more different kinds of fibers in order to satisfy all properties required. For instance, flame-retardant clothes for children are required to have properties such as superior hand touchness, hygroscopicity, durability and wash and wear property in addition to a superior flame-retardant property. However, it is impossible to satisfy all properties mentioned above by employing only a matrix fiber which is made of polyvinyl chloride (hereinafter referred to as "PVC") and polyvinyl alcohol (hereinafter referred to as "PVA") as main polymer ingredients. For this reason, an attempt to blend the matrix fiber and other fiber such as polyester fiber or cotton fiber was made. However, the present inventors have found out that, in case of blending a flame-retardant fiber and a combustible fiber including the above-mentioned case, the obtained composite fiber does not show any flame-retardant property or, even if it shows some flame-retardant property, the property does not stand at all the provision of DOC-FF 3-71 which is the standard with respect to flame-retardant clothes for children in the United States of America.
The above fact will be explained more clearly in the description hereinafter. In the Japanese Patent applications of the assignee of this application, there is described that a matrix fiber which is made of a halogen-containing polymer and PVA (or acetalized PVA) as main polymer ingredients and contains stannic acid in a specific ratio per the amount of the main polymer ingredients is a durable, highly flame-retardant fiber (see Japanese Patent Publication No. 10823/1974 and Japanese Patent Disclosure No. 35623/1974). Even if the conventional matrix fiber per se has a superior flame-retardant property, the flame-retardant property of a composite fiber prepared by blending the matrix fiber and a polyester fiber is extremely lowered, as compared with that of the matrix fiber per se. That is, it is expected that, even if the flame-retardant property [LOI (Limited Oxygen Index) value] of the composite fiber consisting of the matrix fiber having the composition of chlorine-containing polymer: partially acetalized PVA: stannic acid = 49:51:3 by weight and the polyester fiber is lowered with the increase of the proportion of the polyester fiber, the lowering is proportional to the increase of the proportion of the polyester, as shown by Straight line C in Drawing. However, in fact, the flame-retardant property of the composite fiber is lowered more greatly than expected, as shown by Curve B in Drawing.
As described above, there has never been found out a composite fiber capable of satisfying a flame-retardant property and other properties required by consumers, and the development of such a composite fiber has been demanded strongly.